1. Field of the Invention
The present invention relates to mixtures, polyester compositions, films and procedures for their manufacture.
2. Description of Related Art
Aromatic polyesters such as, for example, polyethylene-terephthalate are synthetic materials widely known in the plastics industry which are used, amongst other applications, for the manufacture of films. The economic importance of these synthetic materials requires the use of the most cost-beneficial procedure possible for their preparation. In order to increase the speed of manufacture of the films a procedure is often used which involves the polyester melt carrying an electrostatic charge. This requires that the polyester melt possesses the lowest possible level of specific resistance to achieve a high product quality. This is often achieved by adding metal salts to the melt. For example, EP 1 213 123 describes the influence of different metallic salts upon the product quality of extruded films.
These polyester films have the disadvantage that the synthetic material suffers from low stability. To improve this level of stability phosphorus compounds are most frequently employed. If conventional phosphoric acid derivatives are used, turbidity develops which is disadvantageous for the optical appearance of the films.
Polyesters are known from EP 0 403 664 which contain phosphonium compounds in addition to alkali- and alkaline-earth metal salts. These films exhibit a high degree of transparency. Furthermore, these films possess high thermal stability and they can be produced at a high speed when manufactured by the above-mentioned procedure. There is, however, a disadvantage in that phosphonium compounds are often injurious to health and are able to migrate out of the film. Furthermore, phosphonium compounds are very expensive.
Generally speaking, it is necessary that all the phosphorus compounds used for the application in question be granted FDA—(Food and Drugs Administration) approval.
Beyond this, the publication “Improved Electrostatic Pinning Using a Biased, Conductive Shield” by M C Zaretsky, J E Benson, in the Journal of Electrostatics, 40 & 41 (1997) 735-740) describes the extrusion characteristics of compositions which in addition to metal salts also contain the phosphorus compounds described in the document EP 0 403 664.